Industrial furnaces and steam generators operate at temperatures from 1800 deg. F. to 3000 deg. F., mostly with a gaseous fuel and with combustion air preheated in waste heat regenerators or recuperators up to and above 1500 deg. F. temperatures. The predominant fuel is natural gas burned mostly in diffusion flames. The produced peak flame temperatures cause high emissions of nitric oxide (NO), a product implicated in smog and acid rain problems.
In a typical diffusion flame the fuel and the combustion air discharged from the burner remain separated until brought into intimate contact with one another in a flame zone in the furnace. In a typical low NOx burner the fuel is introduced from a fuel gun located in the centre of the burner as a full cone fuel jet, the combustion air from one or more concentric annual nozzles as hollow cone coaxial air jets enveloping the fuel jet. As a result, several reaction zones are formed in the flame as well as between the flame and the furnace chamber walls.
In a reaction zone formed between the flame and the furnace wall the reactions mostly include the combustion air oxygen and nitrogen and some recirculated combustion products. The occurring reactions are mostly oxidative and are limited by the prevailing temperature which in turn is affected by the radiative thermal energy of the diffusion flame. In the vicinity of the flame there are three reaction zones.
The first--precombustion zone, is a region located on the surface of the fuel jet where the reactions include the fuel just before mixing of the fuel with air. The occurring reactions are mostly pyrolytic, are driven by thermal energy and the reaction products include unsaturated species and particulate nuclei.
The second--flame zone, is a boundary region surrounding the fuel jet. Since fuel is initially separated from the surrounding oxidant, the ratio of the fuel to oxidant in this zone varies in the radial direction from the rich to the lean limit. Regardless of the temperature in this zone, ignition and combustion reactions can occur only after the fuel and the oxidant produce a combustible mixture. Occurring reactions are influenced by the diffusion rates of reactants, by physical processes affecting mixing of the reactants, by heat transfer processes affecting exchange of thermal energy and by the geometry of the flame zone affecting the heat transfer and the residence time of reactants. Reaction products may include particulate as well as atomic, molecular, free radical and anionic gaseous species.
The third--post flame reaction zone, is a region located downstream of the flame zone and is characterized by physical and chemical processes occurring between the reactants. Occurring reactions may be heterogeneous as well as homogeneous, either oxidative or pyrolytic and affect the composition of the combustion products leaving the furnace.
The emission of NOx from combustion systems depends on the quantities of NOx formed and decomposed in the various reaction zones of the diffusion flame and furnace, which depend on the operating conditions of the burner-furnace system. It is generally accepted that formation of NOx in gas fired combustion systems occurs by two mechanisms known as the thermal NO and the prompt NO.
The thermal NO refers to high temperature reactions of nitrogen and oxygen discovered by Zeldovich and is thought to occur mainly in the post flame zone and in the zone between the flame and the furnace walls.
The prompt NO mechanism was first suggested by Fenimore whose measurements showed that some NO is formed at relatively low temperatures in the flame zone through reactions involving radicals C2, C2H, CH and CH2 with air nitrogen and oxygen.
Both NO formation mechanisms are known to depend on the peak flame temperature and on the concentration and residence time of reactants in the reaction zones of the furnace and the described three reaction zones of the diffusion flame.
To minimize formation of NO and to reduce the emission of NOx from combustion systems, numerous studies were devoted to geometries and operating conditions of burners and furnaces. Low excess air firing, flue gas recirculation, fuel and air staging of burners and or furnaces were found to be effective in controlling the formation of the thermal NO, and addition of a suitable diluent to the fuel prior to combustion in controlling the formation of the prompt NO. Still, the prior art burners when operating with high temperature preheated combustion air produce NO at levels requiring expensive post combustion control techniques to meet the late low NOx emission limits.
Therefore, to reduce the emission of NOx from combustion systems operating with preheated combustion air without the need for the expensive post combustion treatment of flue gases, it is the object of the present invention to provide a burner that would operate effectively with fuels and high temperature preheated combustion air and would effectively minimize formation of NOx in the various burner-furnace systems.
It is another object of this invention to improve the geometries of reaction zones in the furnace and in the flame to provide more favourable conditions for the occurring reactions and for minimizing the formation of the prompt NO and the thermal NO.
Another object of the present invention is to provide a burner that would improve the thermal operating conditions and the post combustion reactions in the furnace.
Another object of this invention is to provide a burner that would improve mixing conditions and transmission of unreacted species of hydrogen and carbon monoxide into the post flame reaction zones to improve the destruction of NO therein.
Another object of this invention is to provide a burner that would permit reliable start up and operation of the furnaces and steam boilers from ambient conditions and that would be simple, easy to install and maintain and suitable for retrofitting in furnaces and steam boilers without adversely affecting their heat transfer characteristics.